Field of the Invention
The present invention relates to a heat fixing apparatus and a grease composition for a heat fixing apparatus. Priority is claimed on Japanese Patent Application No. 2014-094209, filed on Apr. 30, 2014, the content of which is incorporated herein by reference.
Description of the Related Art
In image forming apparatus such as electrophotographic apparatus, so-called copying machines, and printers, hitherto, heat roller-type heat fixing apparatus have been widely used for applying heat and pressure to fix, onto a recording material such as a sheet, an unfixed toner image borne on the recording material.
From the standpoint of quick start and saving energy, film heating-type heat fixing apparatus and electromagnetic induction heating-type heat fixing apparatus, which cause films themselves to generate heat, have also been put to practical use in recent years.
Film heating-type heat fixing apparatuses are disclosed, for example, in Japanese Patent Application Laid-Open No. S63-313182 and Japanese Patent Application Laid-Open No. H04-044075.
The film heating-type heat fixing apparatus includes a heater as a heating member, a fixing film as a flexible rotary member that is brought into contact with the heater and rotates while applying heat, and a pressure roller as a pressurizing member that forms a fixing nip portion with the heater via the fixing film.
In the film heating-type heat fixing apparatus, a recording material bearing thereon an unfixed toner image is introduced between the fixing film and the pressure roller at the fixing nip portion and conveyed together with the fixing film while being sandwiched. Accordingly, the unfixed toner image is fixed onto the surface of the recording material by the pressure of the fixing nip portion with the application of heat from the heater via the fixing film. In this heat fixing apparatus, low heat capacity members are used for the heater and the fixing film, and it is sufficient if the heater, which is a heat source, is energized only at the time of executing image formation to generate heat of a predetermined fixing temperature. Accordingly, the heat fixing apparatus has advantages of a short waiting time from power-on of the image forming apparatus to entry into a state in which image formation can be executed and substantially small power consumption at the time of standby.
Japanese Patent Application Laid-Open No. 2003-045615 discloses a metal sleeve for heating, in which a cylindrical metal element tube is used as a base layer and a release layer is provided on the outer surface. In addition, Japanese Patent Application Laid-Open No. H10-010893 discloses a fixing belt, in which a heat resistant elastomer layer is formed on the outer surface of a metal or heat resistant plastic tube and further, a layer of silicone rubber or fluororesin is formed on the outer surface of the heat resistant elastomer layer.
The use of a metal, which has higher heat conductivity than a resin, in place of a hitherto used heat resistant resin, such as polyimide, for a base layer of the fixing film increases the heat conductivity of the fixing film itself, and accordingly, heat from the heater is more efficiently transferred to the recording material. Therefore, it is possible to accommodate the increase in speed of image forming apparatus by using the metal for the base layer of the fixing film. In addition, a fixing film in which the metal is used for the base layer has sufficient strength, thus resulting in an increase in durability and robustness.
Hitherto, fixing unevenness partially occurs in some cases because the surface of the fixing film cannot follow the shape of a toner layer, which is formed by multiply transferring images, when the toner image passes through the fixing nip portion. Fixing unevenness may appear as gloss unevenness of an image, or may lead to transparency unevenness in the case of OHTs (transparent sheets for overhead projectors) and the transparency unevenness may appear as an image defect when projected. To deal with this problem, an elastic layer is provided on the base layer of the fixing film so as to render the surface of the fixing film deformable along the toner layer. Therefore, when the elastic layer is provided as just described, heat is transferred from the fixing film to the toner layer arranged unevenly on an image in such a manner that the heat is enclosed by the fixing film, thereby achieving uniform fixing performance.
On the other hand, Japanese Patent Application Laid-Open No. H08-016005 discloses an electromagnetic induction heating-type heat fixing apparatus in which, with magnetic fluxes, eddy currents are induced in a film member and Joule heat of the eddy currents heats the fixing film itself. The heat fixing apparatus is able to directly heat the fixing film by using the occurrence of induced currents, and achieves a more highly efficient fixing process compared to a heat roller-type heat fixing apparatus having a halogen lamp as a heat source.
In electromagnetic induction heating-type heat fixing apparatus, a thin metal is often used for the base layer of the fixing film. Further, in the case where an electromagnetic induction heating-type heating fixing apparatus is used in a color image forming apparatus, a fixing film having an elastic layer provided on the base layer may be used.
In heat fixing apparatus using the fixing film described above, a lubricant is interposed between the fixing film and the heater or a sliding member, thereby reducing sliding friction between the fixing film and the heater or the sliding member and smoothing the rotational motion of the fixing film.
Both ends of the fixing film are regulated with fixing flanges so that a position in a longitudinal direction of the fixing film and a position in a direction intersecting a conveyance direction thereof are regulated. In some cases, a lubricant configured to reduce sliding friction is interposed also between sliding surfaces of the fixing flange and the fixing film.
Further, the lubricant is similarly interposed in a sliding portion located in the heat fixing apparatus, for example, between an axis of the pressure roller and a bearing that supports the axis, and thereby rotational motion is smoothened.
Because the heat fixing apparatus may be used under an elevated temperature of 180° C. or more, as the lubricant, a fluorine-based grease composition is adopted which shows excellent stability even under high-temperature environments. The basic constituents of the grease composition are base oil and a thickener and the grease composition is formed of perfluoropolyether oil (PFPE) as the base oil, a polytetrafluoroethylene (PTFE) homopolymer or copolymer as the thickener, and an added material, such as a small amount of a rust preventive, as an additive.
The perfluoropolyether oil is roughly classified into a straight-chain type and a side-chain type, and the straight-chain type has small temperature dependence of kinetic viscosity compared to the side-chain type. That is, the straight-chain type has viscosity under a low-temperature environment lower than that of the side-chain type and has viscosity under a high-temperature environment higher than that of the side-chain type. From the viewpoint of reducing driving torque required for activation from a state in which the lubricant is cold under a low-temperature environment, it is preferred that the lubricant to be used in the heat fixing apparatus have low viscosity under a low-temperature environment. This is because the lubricant having low viscosity under a low-temperature environment enables the fixing film to rotate easily. On the other hand, from the viewpoint of suppressing the outflow of the lubricant from the end of the fixing film and the depletion of the lubricant from the sliding friction portion caused by the outflow in the case where the lubricant is used at high temperature, for example, during continuous printing, it is preferred that the lubricant have high viscosity under a high-temperature environment.
In a related-art heat fixing apparatus, perfluoropolyether oil of the straight-chain type having a chemical structure represented by the following structural formula (1) is used as the lubricant.

However, in recent years, there is an increasing demand for the increase in speed and downsizing of a laser beam printer. In order to perform heat fixing treatment in an image forming apparatus at a higher speed, heat energy and pressure force higher than ever are required. Further, there is also a high demand for longer life, and a period of time during which the lubricant to be used in the heat fixing apparatus is exposed to high temperature is becoming long.
Therefore, in the case where the conventional perfluoropolyether oil of the straight-chain type is used as the lubricant, when the rotational motion of the fixing film is continued for a long period of time, the base oil in the lubricant may be evaporated to be depleted. Due to the depletion of the base oil, the slidability between the fixing film and the heater or between the fixing film and the fixing flange is lost, with the result that malfunction such as the breakage of the heater or the breakage of the fixing film may be caused.
Similarly, when the lubricant applied between the pressure roller and the pressure roller bearing is depleted within the life of a product to cause the bearing to be scraped off, abnormality may occur in the rotation of the pressure roller to cause, for example, a conveyance failure of a recording material.
Further, a part of the base oil evaporated in the heat fixing apparatus is conveyed along a conveyance path together with the recording material and decreased in temperature to be liquefied in the conveyance path, and thus there is a risk in that the base oil may adhere to various places on the conveyance path. For example, when the base oil adheres to a recording material conveyance roller on a downstream side of the heat fixing apparatus, the adhering base oil decreases the friction coefficient of the roller to have an adverse effect on conveyance performance. Therefore, the adhesion of the base oil has become a serious problem in ensuring the reliability of the heat fixing apparatus and extending the life thereof.
When the amount of the lubricant is increased so as to prevent the depletion of the lubricant, the amount of components adhering to the inside of the conveyance path is also increased. Therefore, although the problem of the depletion of the lubricant is solved, another problem such as the degradation in conveyance performance may be caused.
The present invention is directed to providing a heat fixing apparatus that suppresses the depletion of a fixing lubricant at higher temperature and the adhesion thereof to a conveyance path and that has long life and high reliability with the same amount of the lubricant as that of the related art.
The present invention is also directed to providing an image forming apparatus and an electrophotographic apparatus that contribute to form a high-quality electrophotographic image.
The present invention is also directed to providing a grease composition that can be used in the heat fixing apparatus.